1. Field of the Invention
The invention relates to cooling superalloy components during welding fabrication or repair, so as to reduce likelihood of weld heat affected zone cracking during weld solidification and during post weld heat treatment. More particularly the invention relates to cooling superalloy steam and gas turbine components, such as turbine blades or vanes during weld repair.
2. Description of the Prior Art
“Structural” repair of gas turbine or other superalloy components is commonly recognized as replacing damaged material with matching alloy material and achieving mechanical properties, such as strength, that are close to the original manufacture component specifications (e.g., seventy percent ultimate tensile strength of the original specification). For example, it is preferable to perform structural repairs on turbine blades that have experienced cracks, so that risk of further crack growth is reduced, and the blades are restored to original material structural and dimensional specifications.
Structural repair of nickel and cobalt based superalloy material that is used to manufacture turbine components, such as cast turbine blades, is challenging, due to the metallurgical properties of the finished blade material. The finished turbine blade alloys are typically strengthened during post casting heat treatments, which render them difficult on which to perform subsequent structural repair welding. For example, a superalloy having more than 6% by weight percentage aggregate aluminum or titanium content, such as cast nickel alloy 247 (Mar-M-247, CM 247), is more susceptible to post heat treatment strain age cracking when thereafter subjected to high-temperature welding than a lower aluminum-titanium content X-750 superalloy.
Currently used welding processes for superalloy structural fabrication or repair generally involve substantial melting of the substrate adjoining the weld preparation site (the weld heat affected zone), and complete melting of the welding rod or other filler material added. When a blade constructed of such a material is welded with rods of the same or similar alloy, the blade is susceptible to solidification (aka liquation) cracking within and proximate to the weld, and/or strain age (aka reheat) cracking during subsequent heat treatment, processes intended to restore the superalloy original strength and other material properties comparable to a new component. Known laser cladding repair processes utilize laser energy to melt filler material and a smaller zone of the blade or vane substrate than comparable traditional welding processes, but often require multiple sequential layer applications to restore eroded substrate surface topology. A laser clad substrate still has a weld heat affected zone, and heat absorption is cumulative during the lengthy sequential cladding process. It is preferred during welding, if possible, to limit the weld heat affected zone temperature to below approximately 572° F. (300° C.), which reduces substantially the likelihood of substrate/weld solidification and reheat cracking.
In the past, chiller block heat sinks have been applied to the superalloy blade or vane turbine component, in order to transfer heat out of the weld zone. Heat transference out of the weld zone reduces the weld heat affected zone volume as well as its peak temperature. Known chiller blocks are generally fabricated from highly conductive metals, such as aluminum or copper, and often incorporate circulating cooling fluid passages. The chiller block heat sinks have surface profiles matching that of corresponding portions of the original manufactured dimensional component substrate surface area, adapted for abutting contact with the substrate component. It is intended to conduct heat from the component substrate surface directly to a corresponding abutting portion of the heat sink. Good conductive heat transfer efficiency requires direct contact between the heat sink and substrate. Manufacturing dimensional tolerance variances in either the component substrate surface or the chiller block surface increases likelihood of non-abutting surface mismatch gaps, which reduces conductive heat transfer efficiency across the gap.
Repairable turbine superalloy components that have been in used in field service often have rough surface finishes and mechanical surface distortion that inhibit full contact between the original factory dimensional profile heat sink and the now field degraded profile component substrate. Dimensional gaps between the component surface and mating corresponding heat sink surface greatly reduce thermal conductivity. Gasses occupying the gaps, such as ambient air, or inert welding gasses also have low thermal conductivity. Gap pads may be inserted between the chiller block heat sink surfaces and the substrate mating surface to help accommodate thermal contact. The gap pads also have limited thermal conductivity compared to direct chiller metal to superalloy contact. Inconsistent and/or asymmetrical gap pad placement also can cause inconsistent cooling profiles across the substrate.
Thus, a need exists for a heat sink apparatus for cooling a superalloy component during component welding that facilitates efficient, uniform conductive heat transfer from the component substrate.
Another need exists for a heat sink apparatus for cooling a superalloy component during component welding that has setup ease comparable to existing known chiller blocks.
Yet another need exists for cooling system for cooling a superalloy turbine component during component welding that facilitates controlled heat transfer, and preferably selectively variable heat transfer rate and capacity in response to the substrate's temperature during a welding process.